In many welding processes it is desirable to provide an ac welding current. For example, both shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), also called tungsten inert gas (TIG) welding, are often performed using an ac welding current. GTAW is a well known method for welding metals, in particular aluminum and magnesium, and a description of GTAW, including the problems associated with welding aluminum and magnesium, may be found in Welding Handbook, Chapter 3, Gas Tungsten Arc Welding, pages 74-107, 1991, published by the American Welding Society.
Phase controlled ac power supplies are known in the prior art. That is, the output current magnitude is controlled by varying the time at which electronic switches are closed with respect to the zero crossing of the input signal. Some prior art phase controlled welding power supplies provide an ac square wave output, having the frequency of the line. For example, U.S. Pat. Nos. 4,038,515 issued to Risberg, which is hereby incorporated by reference, and 4,371,776, issued to Winn, as well as Japanese Kakai 52-33855 assigned to Okada disclose such power supplies. Another type of prior art power supply is shown in PCT patent publication PCT/DE/91/00012, invented by Bodewigs. This power supply provides a substantially square wave output having a frequency that is one and one-half times greater than line frequency. FIGS. 1-4 show the basic power supplies of the '515 patent, the '776 patent, the Okada publication and the Bodewigs publication, respectively, but modified to be a reduced open circuit power supply in accordance with the present invention.
Other prior art phase controlled power supplies produce a sinusoidal output at the line frequency. One such power supply is disclosed in U.S. Pat. No. 5,187,428, which is hereby incorporated by reference. Still other prior art ac power supplies utilize an inverter to produce an ac output. This sort of power supply uses electronic switches to invert a dc signal, thereby providing an ac square wave output. Inverter power supplies often include a rectifier to produce the dc signal from the ac line input.
It is also well known in the art that in ac GTAW the arc extinguishes with every current reversal. When the reversal is to the work piece emitting (electrode negative), it is particularly difficult to reignite the arc. The work piece is generally below thermionic temperature. Also, the molten weld pool is very clean metal due to the ionic bombardment during past electrode positive half cycles. Both of these factors contribute to difficulty in reestablishing the electrode positive half cycle.
Because a higher open circuit voltage helps reignite (stabilize) an arc, it is desirable when welding with ac current to have the output voltage of the welding power source considerably higher during no load (open circuit) than during load (when the arc is present). Indeed, generally speaking, the greater the ratio of open circuit or no load voltage to welding or load voltage the smoother, easier and generally better a power source will operate. Thus, many of the prior art power supplies described above provide a higher open circuit voltage to help strike the arc. In the case of ac welding power sources the maximum allowable open circuit voltage (OCV) has been set by standards at 80 volts ac rms.
It is well known that an increased open circuit voltage is more dangerous. Thus, while 80 volts ac open circuit does assist in starting and stabilizing the arc, the operator must exercise considerable caution because of the potential that exists for a possible electric shock.
In the past there have been many attempts at providing a safer open circuit voltage. One is to use dc current to weld rather than ac because of the greater safety margin of dc. According to the International Electrotechnical Commission's (IEC) Publication 479-1 the ratio of a direct current to its equivalent rms value of alternating current having the same probability of ventricular fibrillation is 3.75 for shock durations longer than the period of one cardiac cycle.
Moreover, according to IEC 479-1 there is an additional safety advantage to dc current over ac current in the "let go threshold." The ac current level above which a person holding electrodes cannot let go of the electrodes is on the order of 10 milliamps for ac currents in the range of 15 to 100 Hz. While the exact let go threshold for dc current has not been established, it is considerably greater (greater than 300 Ma) than that of ac current.
While there is a safety incentive to use dc current for welding, many welding applications require ac current. For example, in places where "arc blow," the influence of a magnetic field to misdirect the arc, is a problem or when GTAW needs to be performed on refractory oxide metals such as aluminum and magnesium alloys, ac is often required. Thus, ac power supplies such as those described above are necessary.
In some circumstances the welding operator can be adequately protected from a hazardous ac shock by the use of dry insulating clothing (especially gloves), and by using insulating mats or boards to eliminate contact with the work piece.
However, in environments that are wet, physically confining, tight, cramped or hot where sweat can permeate clothing and gloves such safety precautions can be difficult or impossible. In such environments safety standards recommend the use of devices that reduce the open circuit voltage of welding power sources to a lower value while not welding. Some examples of prior art in this respect can be found in U.S. Pat. Nos. 4,450,340; 2,444,168; 2,769,118; 2,775,735; 4,015,188; 2,502,646; 2,960,628; 2,449,456; and 2,617,913.
Generally speaking, the prior art devices for reducing open circuit voltage (OCV) have been unsatisfactory. One of the primary reasons has been cost. The additional components necessary to accomplish OCV reduction in accordance with the prior art can increase the cost of the product significantly. Since most work is not done in the hazardous environments described above, the feature is often not ordered where it is an extra cost option or where a less expensive power source is available where this is not a standard feature.
Additionally, add-ons have not been popular because they get in the way, require extra wiring, are vulnerable to abuse, costly and require special or modified electrode holders. For examples of OCV reduction add-ons see U.S. Pat. Nos. 2,936,365 and 4,151,396.
Prior art OCV reduction devices often require a significant delay before restoring the high voltage. However, a delay of more than a few cycles before restoring high voltage may be noticed by an experienced operator. Also, when the welder makes electrode contact with the work piece it is done with a very brief tap or the electrode will weld itself to the work piece making the welder tug and wiggle the electrode to break the weld. During this crucial starting period the welder requires high current instantly or it creates a functional problem for him. Thus, to work satisfactorily an OCV reduction device must be very quick and it must be sensitive to the electrode to work impedance which indicates when the electrode contacts the work.
Moreover, the prior art OCV reduction devices required expensive added components. Such components, especially those required to conduct high current, require frequent maintenance and increase the possibility of failure.
Accordingly, there is a need for an ac welding power supply that includes a device to provide a reduced open circuit voltage. Preferably, such a device would provide a dc open circuit voltage to take advantage of the dc current safety advantage with respect to both ventricular fibrillation and the "let go threshold."
Additionally, there is a need for an OCV reduction device that reduces the OCV in ac welding power supplies, including phase controlled and inverter ac power supplies, yet still provide a dc, electrode positive, OCV. Preferably such an OCV reduction device would be economical and not an add-on. It should restore the high voltage without a significant delay and be sensitive to the electrode to work impedance. Finally the OCV reduction device should not require expensive added components and should be reliable.